This invention relates to an imaging system and, more specifically, to lithography.
Lithographic printing is a well-known and established art. In general, the process involves printing from a flat plate depending upon the difference in properties between image and non-image areas for printability. In conventional lithography, the non-image area is hydrophilic while the image is hydrophobic. A fountain solution is applied to the plate surface which wets all portions of the surface not covered by the hydrophobic image. This solution keeps the plate moist and prevents it from scumming up during the printing phase of the process. An oil-based printing ink is applied to the image surface, depositing the lithographic ink on the image area, the hydrophilic non-image area repelling the ink. The ink image may then be transferred directly to a paper sheet or other receptive surface, but generally it is transferred to a rubber offset blanket which, in turn, transfers the print to the final paper copy. Hence, for each print made during a run, the lithographic plate is first dampened with an aqueous fountain solution, inked with a lithographic printing ink and printed via an offset blanket onto the final receptive copy sheet.
It has been known that lithographic plates can be made electrophotographically by utilizing conventionally developed electrophotographic plates as lithographic printing masters. In these systems, usually a zinc-oxide type of plate is charged by conventional means and exposed to the image to be reproduced with the resulting electrostatic latent image developed with conventional electrostatic toner. The toner is generally hydrophobic in nature, as is the undeveloped background area of a conventional binder-type electrophotographic plate. In order that the developed plate be useful as a lithographic master, a differential must be established between the toner image and the background of the plate. Since both are hydrophobic in nature, it is necessary to treat the background of the electrophotographic plate by the use of a conversion solution so as to render the background surface hydrophilic in nature. After the alteration of the non-image, background area, a nonaqueous, oil-based ink can be used whereby the toner will accept the ink and the now hydrophilic background areas will repel the ink.
While these systems have been found useful for lithographic purposes, there are inherent disadvantages in their use. For example, in the preparation of a printing master, it is necessary, in order to produce final printed images of lithographic quality, that the developed electrophotographic image be of extremely high-quality copy with sharp images of high-image density and minimal background so that the master produced can withstand the vigors of the lithographic process over extended periods of usage, a lithographic system inherently being a high-volume printing process. In the heretofore used developer systems in preparing electrophotographic printing plates to be utilized in lithographic printing processes, the images formed have been found to be less than adequate to produce the results required in lithography. Poor quality images have led to deficient masters which produce relatively short periods of usage time. In addition, high background and poor image density have contributed to the deficiencies.
It is, therefore, an object of this invention to provide a lithographic printing system which will overcome the above and other disadvantages.
It is a further object of this invention to provide a novel method for the preparation of a lithographic printing master.
Another object of the present invention is to provide an imaging system utilizing a novel lithographic master prepared from an electrophotographic plate.
Still a further object of the present invention is to provide a novel lithographic printing plate utilizing electrophotographic principles.
Yet, still a further object of the present invention is to provide a lithographic printing plate prepared by an electrophotographic system wherein the master produced is of a high-quality image with low background and high-image density and sharpness.
The foregoing objects and others are accomplished in accordance with the present invention, generally speaking, by providing a lithographic printing plate prepared electrophotographically wherein a single component developer composition, hereinafter referred to as a developer toner, for developing electrostatic latent images, is utilized comprising a resinous component having a sharp melt point and low melt viscosity essential for heat fusing and exhibiting good melt-flow characteristics inclusive of superior wetting properties in a short dwell time, heat-fusing environment. For purposes of the present invention, polyamide resins were determined to be highly suitable for use as the single component toner resinous component due to the sharp melting point characteristics and low melt viscosity of the polyamide, which is essential for short residence time heat fusing. Included as a component of the polyamide toner composition is a magnetic oxide material, generally present in the toner composition in an amount of from 40 to 75 percent by weight of the instant developer composition. The resulting polyamide-magnetic oxide toner composition possesses excellent melt-flow characteristics in that it exhibits a sharp melting point and low melt viscosity and flows evenly so as to become congruous with the substrate. Since the subject resins have good melt-flow characteristics, they inherently possess the capability to desirably wet out the highly loaded magnetic oxide particles in the formulation. This wetting-out characteristic and relatively good melt-flow property of the toner is also attributed to the presence of the specific magnetic oxide selected since the magnetic oxide has good dispersing characteristics and plays an important role in the melt-flow mechanism. The developer toner further preferably includes a highly conductive carbon pigment to regulate the resistivity of the resulting toner particle.
It has been determined in the course of the present invention that lithographic printing masters may be fabricated electrophotographically so as to produce a master having an extremely sharp image with minimal background utilizing a single component developer composition or toner comprising a pigmented polyamide resin and a magnetic oxide component to develope the electrostatic latent image. The instant developer composition exhibits the necessary characteristics which permit the toner to be used in a pressureless, heat-fusing electrophotographic imaging process. The resulting printing masters fabricated are more durable with respect to normally encountered environmental conditions and handling without the toner images blocking or adhering to one another, for example, in or under normally used shipping conditions. Sharper melting type resins reduce cold flow tendency in that they do not tend to soften until the environmental temperature closely approaches the melting temperature. The utilization of the polyamide resin of the present invention will permit the formulation of a single component toner with a magnetic oxide content ranging from 40 to 75 percent by weight. The low melt viscosity property of the resin aids in the ability to fuse the toner adequately at surface-fusing temperatures of 215.degree. to 225.degree. F., at fusing rates of 3.8 inches per second. It is generally known that heat-fusible toners usually provide for a cleaner background since pressure-fusing rolls, heretofore used in conjunction with single component toners, tend to move the toner particles about on the surface supporting the electrostatic latent image, thus enlarging background particles as pressure is applied. The heat-fusible toners of the present invention are especially suitable for fabricating lithographic masters herein defined which are subsequently used for duplicating. The polyamide resins used in the process of preparing the printing master of the present invention provide for the required sharp melting point and low melt viscosity of 1,000 centipoises or less of the resinous constituent, thus providing the desirable melt-flow characteristics when used in combination with the magnetic oxide additive in developing the electrostatic latent image.